Systems have been proposed to enable printed substrates to be used interactively, in a manner similar to how computer displays are used interactively. By “interactively”, we mean that the printed substrate includes visible human-readable information which, when “activated” by the user using an appropriate device, causes an action to occur in the system. Systems have been proposed in which pages are printed with machine-readable tags that are not discernable to the unaided eye of a human. These tags are typically printed over the entire surface of each page in a regular pattern. Each tag incorporates information, hereinafter referred to as a page ID, that differentiates each page from other pages and information, and information, hereinafter referred to as a location ID, that differentiates a subarea of the page associated with the tag from other subareas within each page. The association between the machine- and human-readable information is recorded in the system, and the system is thereby able to interpret actions performed relative to the substrate by the user using an appropriate tag-sensing device as actions relative to the human-readable information.
In our earlier application U.S. Ser. No. 09/575,187, we have proposed a printer which prints both the machine- and human-readable information simultaneously and a system that has the ability to print customized versions of documents, for example newspapers, for different customers. When the same printer prints the machine- and human-readable information, it is relatively simple to associate unique page and location IDs with the human-readable data. However, where pages are used with pre-printed or pre-formed tags it is necessary to associate each page with the data printed on a page-by-page basis. Even with mass production there is no certainty that a particular paper sheet will have particular information printed on it or that particular information will be printed on a sheet having a particular page ID or sheets having a particular range of page IDs.